Blade transition for axial-flow compressors and the like

ABSTRACT

A transition of improved design for joining the merging surfaces of the airfoil and platform of an axial-flow compressor blade of the cantilevered type. The transition as viewed in profile is a line comprising (a) an essentially straight major portion diverging outwardly from and faired into the airfoil and (b) an outwardly concave minor portion extending along and faired into the platform.

United States Patent Hendrix et a1.

BLADE TRANSITION FOR AXIALFLOW COMPRESSORS AND THE LIKE Inventors: Donald E. Hendrix; Richard E.

Ziegler, both of Oakridge, Tenn; Weldon E. Swinson, Auburn, Ala.

The United States of America as represented by the United States Energy Research and Development Administration, Washington, DC

Filed: June 28, 1972 Appl. No.: 266,900

Assignee:

U.S. Cl 416/234; 416/223 Int. Cl. F04d 29/38 Field of Search 416/234, 239, 248, 223;

References Cited UNITED STATES PATENTS Parsons 29/1568 8 1 June 17, 1975 1,809,131 6/1931 Maison 416/234 FOREIGN PATENTS OR APPLICATIONS 946,794 1/1964 United Kingdom 416/228 475,199 10/1952 ltaly .1 416/234 502.546 11/1954 Italy 416/234 1.151.228 1/1958 France 416/223 Primary E.raminerEverette A. Powell. Jr. Attorney, Agent, or Firm-John A. Horan; David S. Zachry; F. 0. Lewis [57] ABSTRACT A transition of improved design for joining the merging surfaces of the airfoil and platform of an axial-flow compressor blade of the cantilevered type. The transition as viewed in profile is a line comprising (a) an es sentially straight major portion diverging outwardly from and faired into the airfoil and (b) an outwardly concave minor portion extending along and faired into the platform.

5 Claims, 5 Drawing Figures PATENTEDJUN 17 m5 SHEET BLADE TRANSITION FOR AXIAL-FLOW COMPRESSORS AND THE LIKE BACKGROUND OF THE INVENTION This invention was made in the course of, or under, a contract with the United States Atomic Energy Commission.

This invention relates broadly to improvements in cantilevered blades for axial-flow compressors and the like. More particularly, it relates to cantilevered blades having a novel fillet-region contour which reduces stress concentrations resulting from bending vibrations.

Cantilevered blades for axial-flow fluid machines comprise an airfoil-supporting base, or platform, and a plate-like airfoil which extends therefrom at roughly a right angle. The platform is adapted to be mounted securely to a rotor or stator, and is commonly formed with a threaded shank for mounting. In a normal operation of a blade, the airfoil tends to flex; thus, it is typical for high bending stress concentrations to develop at the two junctions where the faces of the airfoil merge with the surface of the platform. It is not uncommon for blades to fail due to fatigue at these junctions; failure of a single blade often causes the failure of others, with catastrophic results.

It is standard practice, therefore, to very slightly thicken the root, or base, of the airfoil and to contour the surface of the thickened portion to form on either side thereof a concave transition, or fillet, extending between the airfoil and the platform. The design of these blade fillets has an important bearing on the service life of the typical blade. Moreover, the fillets affect operating efficiency by influencing the flow patterns of the fluid being compressed.

SUMMARY OF THE INVENTION It is, therefore, an object of this invention to provide a novel form of airfoil-platform transition for blades employed in axial-flow compressors and the like.

It is another object to provide an airfoil-to-platform transition characterized by reduced principal stresses resulting from bending vibrations.

It is another object to provide an airfoil'to-platform transition which operates with reduced principal stresses and has little or no adverse effect on process fluid flow patterns.

Other objects will be made evident hereinafter.

Our invention can be summarized as follows: In an axial-flow fluid machine blade including a platform and an airfoil extending therefrom, a face of said airfoil merged with a surface of said platform via a transition formed in a thickened root portion of said airfoil, the variation comprising a transition whose surface in profile is a line consisting of (a) an essentially straight major segment sloping outwardly from said face and toward said surface and (b) an outwardly concave minor segment faired into said surface.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view ofan axial-flow compressor blade having a concave-convex airfoil;

FIG. 2 is a profile view (scale, 4:l) of the fillet regions of a blade of the kind shown in FIG. I. Various airioilto-platform transitions are shown, including a conventional circle-arc fillet designated as 19;

FIG. 3 is a similar view ofa blade provided with transitions designed in accordance with this invention;

FIG. 4 is a diagram illustrating that a curved portion d ofa transition designed in accordance with this invention conforms to the quadrant of an ellipse; and

FIG. 5 is a diagrammatic top view of a blade of the kind shown in FIG. 1, the concave side of the blade being uppermost.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, our invention will be illustrated as employed in a conventional axial-flow compressor blade 1 comprising a platform 3 having a substantially flat, circular face 5, from which a centrally positioned, concavo-convex airfoil 7 extends. This airfoil includes a comparatively thick root portion 9 and a thinner tip portion 11. The platform 3 is formed with a base portion 13 having a shank 15. The base portion 13 is tapered for seating in a counterbored hole formed in the wall of the compressor rotor or stator, and the shank 15 is threaded for reception of a nut (not shown) for locking the blade in the seated position. The surfaces of the platform face 5 and the concave face of the airfoil 7 merge in a region which is designated generally as 17 and which is referred to herein as the fillet, or transition, region of the blade. On the opposite side of the blade 1, the surface of the platform face 5 merges with the surface of the convex face of the airfoil in a similar fillet region 17 (FIG. 2).

Referring to FIG. 2, in a normal operation of the blade 1 it is typical for the greatest bending stresses to occur at the junction of the platform and airfoil--i.e., in the regions 17. The bending stresses decrease nonlinearly along the length of the airfoil, from its root to its tip. It has been the practice to reduce bending stresses in either of the regions 17 by slightly thickening the root of the air-foil at its junction with the platform and contouring the thickened portion to provide a circle-arc fillet 19 as the transition between the merging surfaces. The ends of the circle arc extend for about equal distances along the airfoil and the platform, and are faired into the surfaces thereof.

In accordance with our invention, a further significant reduction in principal bending stresses is achieved by providing a transition of novel design between the merging surfaces of the platform and the airfoil. Viewed in profile or in cross section, the surface of the novel transition is a line 21 (FIGS. 2 and 3). As shown in FIG. 3, the line 21 includes an essentially straight major portion 0 and an outwardly concave minor portion :1. The curved portion 0' extends outwardly along the platform and is faired therein. The straight, or ramp," portion 0 is faired into the airfoil. As shown in FIG. 3, the ramp is faired into (is essentially tangent to) the airfoil in a region designated as t and slopes outwardly from the projected face of the airfoil (shown in dashed lines). The slope, or divergence, of the ramp also is referred to herein as a taper.

By tapering the root portion of the airfoil as described, we reduce bending stresses in the regions 17. In accordance with our invention, we design the ramp with a linear taper which varies the moment of inertia and maximum fiber distance from the neutral axis to the extent required to ensure operation with essentially constant surface-bending stresses in a given region 17. That is. the ramp is designed with a slope providing an essentially constant applied-moment-to-sectionmodulus ratio along the length of the ramp. Normally.

for a cantilevered blade of constant cross section, the closer to the attached end (platform end) the greater the bending stress. By designing the blade cross section for constant bending stress in the ramp region, the most efficient use is made of the blade material; that is, a minimum amount of material is employed for the desired reduction in stress. With knowledge of the principle of this invention, one versed in the art can readily compute the taper which, for a specific application, will provide essentially constant surface-bending stresses. The preferred taper for a particular blade will, of course, depend on such factors as blade composition. weight, and cross section. In the case of aluminum alloy blades of the design illustrated in FIGS. 1-2, tapers in EXAMPLE I To compare various types of transitions, scaled-up models of blades of the kind shown in FIG. 1 were prepared for photoelastic frozen-stress analysis. The blades were cast from an epoxy resin and cured under conditions essentially eliminating frozen-in stresses. One lot of models was cast with circle-arc transitions, another with ramp-quadrant transitions, etc.

In the typical test, the shank of a model blade was mounted in a suitable support, the blade extending horizontally with its concave side uppermost. The only load on the blade was that imposed by the weight of the h of about to tu'inch pet inch are espe blade itself. The mounted blade was heated to critical ctatlylsuttable' I temperature and then cooled to freeze-in weight- AS Illustrated we t to (1699 the induced stresses. A grid was projected on the concave j' portlott d of out ttartsttloln as t quadmnt surface to establish the locations of points thereon relaof an elhpse' That the ramp C tmted mm the plat tive to a longitudinal axisy and atransverse axis x (FIG. form 5 by means of a fillet d whose Surface essentially 5), a transverse centerline AB being maintained level conforms, in curvature and length, with the quadrant of with respect to the horizontal. ellipse t23)' AS t be Shown We have found t The models were sectioned into longitudinal slices the resulting compostte rampquadmmi transmon 0.1-inch thick, at depth intervals of 0.3 inch. Using the significantly reduces peak bending stresses in the tran- Tardy compensatinn Method (Revue dioptique v sition region and appreciably narrows the distribution 8, 1929, p 59 ff the maximum fringe Order each of Such Stresses therein The quadrant Pmtion d not side of the airfoil was evaluated for each slice, and its only contributes to these advantages but, in this combilocation recorded In addition, the f i Order at y nfnton at least wt? find i to be todynamicauy Supe' 1.5 inches was determined on each side of the airfoil to cons tanttadtus fillet "t cross for comparative purposes. The table (below) compares Secttona] area Presented to the fund bemg the results obtained with the types of transitions illuspressed, resulting in less blockage of that flow. The dirated in MG. 2 namdy; a circ|e arc 19 FIG. 2 h mansions of the quadrant Portion d can be Varied ing a radius of 5/16 inch; a ramp--circle-arc (21, FIG. within limits consistent with reducing blockage to flow 2 Convex Side), the ramp h i a l h f 1- and with good fittekdestgtl P as regards aVOidtng mately one inch and a taper of 0.08 inch per inch, the abrupt changes in transition cross section. As utilized are having a radius f 5/1 i 3 quadrant f an 1. in aluminum-alloy blades 0f the type illustrated in lipse (19, FIG. 2) having a major diameter of 2.0 inches l 2, the portion dcan be described as conformand a minor diameter f inch; and a ramptng essentially with quadrants 0f etttpses having a quadrant (21, FIG. 2, concave side), the ramp having major-to-minor-diameter ratio in the range ofabout 3:1 3 l h f approximately one i h d a taper f (108 to 411 and mal'ot'ditlmetet length in the range of inch per inch,the quadrant conforming to that ofan e1 a ut -5 I0 2 inches lipse having a major diameter of 1.00 inch and a minor In another embodiment of our transition, the ramp di f (1286 i h, portion is faircd into th p at o m y means of 8 Circle- The table (below) compares the various transitions in arc fittet- This p" transition is terms of stress concentrations at various points, the cotrated on Ihfi convex side Of th Hil'fOii shown 11] rdinates for [he points being hgwn A stress onger 2 and FIG. 3 and s designated as The fiircle-flfc tration is defined herein as the ratio of the maximum portion thereof is designated as 29. In other embOdistress (try) to the surface stress (o'y 1.5) at the locaments of our invention, the fillet of the transition can tion y 1.5 inches.

TABLE Frozen-StressModel Stress Concentrations Blade Circle-Arc Ramp-Circle-Arc Elliptical Quadrant Ramp-Quadrant x Locution Side rryMax/rrylj Location rryMax/(ryl 5 Location rryMax/oyLS Location rryMax/rryLS Location 0.) C 1.00 y=Ll7 1.01 y=L35 1.00 1.02 y=1.01 41.6 0 1.10 \=0.5fi L09 y=Ll7 L36 y=l.02 1.04 y=0.86 0.3 N L79 y=0.50 L57 y=0.48 L34 0.82 1.42 y=0.67

.0 C 2.2) 054 2.00 y=0.22 L52 3 082 1.45 y=0.53 0.3 A 2.22 y=055 2.23 y=038 L46 y=0.73 1.53 y=0.55 0h L40 =0,49 L78 y=0.28 L05 y 0.77 1.35 y=0.62 0.9 E 1.40 048 L44 y 0.48 L09 y=l.13 1.17 y=0.87

Referring again to the table, the circle-arc transition exhibited the highest maximum stress concentrations: 2.29 on the concave side and 1.45 on the convex side. The corresponding values for the elliptical-quadrant transition were 1.52 and L1 7, and for the rampquadrant transition were L53 and l.l3. Thus, compared to the circle-arc, the ramp-quadrant transitions reduced the principal stress concentrations by 33 percent (concave side) and 19 percent (convex side). As shown, both the elliptical quadrant transition and the ramp-quadrant transition exhibited significantly narrower ranges of stress concentrations than did the other designs.

EXAMPLE ll Aluminum alloy blades provided with ramp-quadrant transitions were installed in the first stator row of an axial flow compressor and operated in a test loop. The blades, which were of the configuration illustrated in FIG. 1, were provided with strain gages. The operation of the blades was entirely satisfactory. The blades sustained no fatigue damage and, as determined from the strain gage outputs, operated with reduced stresses from bending-mode vibration. The total stresses for the ramp-quadrant design were approximately half of those for control blades having a circle-arc transition.

It will be apparent that this invention is applicable to cantilevered blades of various sizes and cross-sectional shapes, as well as to cantilevered blades produced by various techniques, such as casting, forgoing, and machining.

What is claimed is:

1. In a finished axial-flow gas machine blade including a platform and an airfoil extending therefrom, a face of said airfoil merged with a surface of said platform via a transition disposed entirely in the fillet region ofsaid airfoil, the said transition defining in profile a line consisting of (a) an essentially straight major segment sloping outwardly from said face and toward said surface, the slope of said major segment having a value providing an essentially constant applied-moment-tosection-modulus ratio along the length thereof, and (b) an outwardly concave minor segment faired into said surface, said minor segment being substantially superimposable on a quadrant of an ellipse.

2. The combination of claim 1, wherein the slope of said major segment, relative to said face, is in the range of about 0.05- to 0. [0-inch per inch.

3. The combination of claim 1, wherein said ellipse has a major-to-minor-diameter ratio of from about 3:1 to 4:1.

4. The combination of claim 3, wherein said ellipse has a major diameter of from about 0.5 to 2 inches.

5. The combination of claim I, wherein said minor segment has an essentially constant radius of curvature. 

1. In a finished axial-flow gas machine blade including a platform and an airfoil extending therefrom, a face of said airfoil merged with a surface of said platform via a transition disposed entirely in the fillet region of said airfoil, the said transition defining in profile a line consisting of (a) an essentially straight major segment sloping outwardly from said face and toward said surface, the slope of said major segment having a value providing an essentially constant applied-momentto-section-modulus ratio along the length thereof, and (b) an outwardly concave minor segment faired into said surface, said minor segment being substantially superimposable on a quadrant of an ellipse.
 2. The combination of claim 1, wherein the slope of said major segment, relative to said face, is in the range of about 0.05- to 0.10-inch per inch.
 3. The combination of claim 1, wherein said ellipse has a major-to-minor-diameter ratio of from about 3:1 to 4:1.
 4. The combination of claim 3, wherein said ellipse has a major diameter of from about 0.5 to 2 inches.
 5. The combination of claim 1, wherein said minor segment has an essentially constant radius of curvature. 